1. Field of the Invention
This invention relates to a torque control system for a hybrid vehicle, which enables a motor to compensate a drop in engine torque when shifting gears of a transmission.
2. Description of Related Art
Japanese Patent Laid-Open Publication No. 11-69509 discloses an example of a conventional torque control system for a hybrid vehicle.
The structure of this hybrid vehicle is similar to that of an ordinary mechanical transmission, which is manually operated, except that there is additionally provided an actuator for use in shifting gears, controlling a clutch, and controlling a throttle of an engine. If it is determined that it is necessary to shift gears according to a preset shift map, a sequence of operations such as closing of a throttle valve, shifting gears, operation of a clutch, and opening of the throttle is automatically carried out.
In such an automatic transmission, drive torque outputted from an engine is temporarily blocked when gears are shifted, and this may give a driver bad impressions such as a sense of deceleration and a sense of free running due to a change in the acceleration.
Accordingly, the hybrid vehicle disclosed in the above publication increases drive torque of a motor to compensate a drop in engine torque when the clutch is determined as being disengaged according to the stroke amount of the clutch.
In the torque control system for the hybrid vehicle as mentioned above, however, the control of the motor torque is started when the clutch is disengaged, and the control of the motor torque is stopped when the clutch is engaged. This results in excessive or insufficient motor torque when the clutch is disengaged or engaged, and it is therefore impossible to prevent a driver from feeling a sense of incongruity.
Supposing that the clutch is disengaged, if the throttle valve is closed prior to the disengagement of the clutch as shown in FIG. 8, a driver feels a sense of deceleration due to the decrease in engine torque ET until the disengagement of the clutch, and feels a sense of free running until an actual motor torque MT is increased to reach a target motor torque tgtMT after the disengagement of the clutch. To address this problem, it may be considered that the throttle valve is closed at the same time as the disengagement of the clutch as shown in FIG. 9, but in this case, the increase in the motor torque is unavoidably delayed. Thus, the momentary deficiency in torque fluctuates the acceleration of the vehicle and causes a driver to feel a sense of incongruity.
It is therefore an object of the present invention to provide a torque control system for a hybrid vehicle, which enables a motor to surely compensate for a drop in engine torque when gears are shifted, thus preventing a driver from feeling a sense of incongruity.
According to the present invention, when engine torque is decreased to shift gears in the transmission, a difference between the actual engine torque and the target torque corresponding to the depression amount of an accelerator pedal is increased to cause the motor torque to increase. This compensates for a deficiency in the actual engine torque with respect to the target engine torque corresponding to the depression amount.
Thereafter, when the transmission of the actual engine torque is terminated due to the disengagement of the clutch, the motor is controlled based on the target engine torque corresponding to the depression amount by a driver, so that the motor torque can compensate all the engine torque corresponding to the depression amount.
Further, when the engine torque increases due to the engagement of the clutch upon completion of shifting, the difference between the actual engine torque and the target engine torque corresponding to the depression amount decreases as the actual engine torque increases, thus causes the motor torque to decrease. This compensates the deficiency in the actual engine torque with respect to the target engine torque corresponding to the depression amount.
Thus, when the engine torque starts decreasing prior to the disengagement of the clutch, the motor torque is controlled to compensate for a deficiency in the engine torque. Likewise, after the engagement of the clutch, the motor torque is controlled to compensate for a deficiency in the engine torque until the increase in the engine torque is completed.